With the need for less expensive and more specific blood typing reagents, laboratories are utilizing plant proteins for the typing of A1 and H-like red cell antigens. In order to obtain a purified well characterized type B erythroagglutinin and D-galactosyl binding protein we are studying the anti-A plus B lectin from Sophora japonica seeds. The purified lectin of 133,000 daltons exists as a tetramer of monomeric polypeptide chains, each possessing a single half-cystinyl residue per 33,500 daltons. The native protein is composed of two dimeric units bound by SDS dissociable bonds but without covalent linkages. Each dimer of 68,000 daltons consists of two polypeptide chains linked by a single disulfide bridge. As determined by analytical ultracentrifugation, polyacrylamide gel electrophoresis and ion-exchange chromatography, the native protein appears to exist as rapidly equilibrating interconvertible conformers of the same molecular weight, but differing in charge distribution. Studies conducted with human erythrocytes of known antigenic composition and untreated and modified blood group substance indicate that the S. japonica lectin displays anti-I activity as well as anti-A and B properties. In addition, employing the hapten inhibition of adsorption technique developed in this laboratory we have obtained evidence for an aromatic binding site adjacent to the saccharide site of the lectin.